1. Field of the Invention
The present invention is related generally to pistons for internal combustion engines.
2. Related Art
In a continuing effort to improve the power production and fuel efficiency of internal combustion engines, the designs of monobloc, or one-piece, piston bodies for internal combustion engines have become increasingly complex to accommodate such features as cooling galleries and uniquely shaped combustion bowls. Often, it is not cost effective to form such complex piston designs from a single unitary piece of material. Therefore, many piston manufacturers form piston bodies of two or more components and subsequently join those components together. For example, some monobloc piston designs include a first piece which includes a crown portion and a second piece which includes opposing skirt portions and opposing pin bores for receiving a wrist pin. The first and second pieces may then be joined together through a range of different joining processes including, for example, various types of welding, bolts, fasteners, etc.
One particularly cost effective approach for joining together the first and second pieces of a monobloc piston body is to friction weld those two pieces together by rotating one piece relative to the other at a high rate of speed and bringing the pieces into contact with one another such that heat from friction between the two pieces causes the material at the juncture to heat to a plastic state. The high pressure of the materials at the juncture has the effect of bonding the pieces together. Subsequent cooling of this melted material has been found to create a particularly strong weld joint between the first and second pieces of the monobloc piston body. However, one issue with this approach is it is very difficult to precisely rotationally align the first and second pieces with respect to one another during friction welding. As such, in general, friction welding is only practical where one (or both) of the first and second pieces is rotationally symmetrical such that its rotational alignment relative to the other piece does not affect the shape of the piston body. Since wrist pin bores, by their very nature, cannot be rotationally symmetrical, piston manufacturers typically overcome this limitation of friction welding by only using friction welding to join the pieces of piston bodies which have rotationally symmetrical combustion bowls.